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CSS
Abbreviation for Cascading Style Sheets
In short and in plain language
CSS is a language that sets how a web page looks.
It sets things like colours, fonts and spacing.
It also has its own unit for sizes on the screen. This unit does not depend on the real screen resolution.
Definition
CSS is the language that defines how a web page looks, such as colours, fonts and spacing. A CSS pixel is a unit for sizes on the screen, independent of the actual screen resolution.